Spinal cord stimulation (SCS) has been used as a treatment for chronic painful conditions for approximately thirty years. Commonly, SCS is used to alleviate pain after failed surgery, pain due to neuropathies, or pain due to inadequate blood flow. SCS originated shortly after the publication of Melzack and Wall's “Gate Control Theory.” The theory essentially states that mutual inhibitory connections exist between slow conducting nociceptive C-fiber sensory neurons and fast conducting non-nociceptive Aβ fiber sensory neurons. Whether or not this mutual inhibition is direct and/or indirect via inhibitory interneurons is not completely understood, but it is known that this inhibitory effect can be seen at the level of the second order dorsal horn neurons that project both nociceptive and non-nociceptive sensory information to the brain. Thus, the therapeutic effect of spinal cord stimulation may act according to the principals of Gate Control Theory or it may be efficacious due to as yet poorly understood mechanisms (e.g., inhibitory interneuron activation, pain perception modulation in the brain). Regardless of the specific mechanism, those skilled in the art have stimulated non-nociceptive fibers as a therapy to alleviate pain symptoms in cases of chronic pain.
In practical application, electrodes are implanted within the epidural space for delivery of electrical stimulation. The electrodes are coupled to a pulse generator which generates high frequency stimulation pulses. Specifically, conventional spinal cord stimulation applies stimulation pulses to neural tissue of the dorsal column in a regular pattern with each pulse being separated by a fixed inter-pulse interval that defines the stimulation frequency. It is believed that high frequency tonic stimulation acts as a “digital lesion” which prevents communication of pain signals to the thalamus of the patient. Specifically, high frequency stimulation has been observed to prevent the perception of certain types of pain by patients. Instead of perceiving pain, the high frequency electrical stimulation causes other sensation signals to reach the thalamus whereby the patient experiences a tingling sensation known medically as paresthesia. Although the paresthesia can be uncomfortable or even painful in patients, the paresthesia is usually substantially more tolerable than the pain previously experienced by the patients and, hence, is considered an acceptable negative side-effect.